You can use macros for this.
set macros
With this enabled (in version 5 by default), you can define string variables which are replaced in a command with their content before the command is executed, like
set macros
Helvetica = 'font "Helvetica, 12"'
Verdana = 'font " Verdana, 12"'
set title "My title" @Verdana
set xlabel "My x-label" @Helvetica
plot x
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